Automatic gain control (AGC) circuits generate a relatively constant output signal amplitude from an input signal with varying amplitude. A typical AGC circuit includes a loop having a variable gain amplifier (VGA). A common application of an AGC circuit is in digital communication systems. An ideal AGC action would provide a constant output for all values of input signal strength. The figure of merit applied to AGC action is given as the change in input required for a given output change.
In high speed (e.g., 10 giga bits per second (Gb/s)), high performance, serial communication receivers that require equalization, VGAs are sometimes used at the front end of the topology. A VGA is used to either provide gain or attenuation depending on the amplitude of the input signal such that the VGA outputs a substantially constant amplitude signal. The ability to adjust the gain/attenuation of the VGA so that both a large and a small input voltage swing range at the input to the receiver can be accommodated is desirable for 10 Gb/s serial data communication applications.
A block diagram of a generic AGC loop 10 is shown in FIG. 1. Amplitude Detector 14 senses the output amplitude Vout 13 of the VGA 12 and generates a voltage that represents the peak voltage of the VGA output Vpk 15. A comparator 17 compares the detected amplitude Vpk 15 to a reference voltage Vref 16. The reference voltage Vref 16 represents the desired output amplitude of the VGA. Based on the comparison, the comparator 17 generates an error signal 18 and feeds it to an AGC loop filter 19. In other words, comparator 17 determines the difference between the peak voltage Vpk 15 and the reference voltage Vref 16, and adaptively adjusts the control voltage Vc 11, such that the VGA 12 produces an output swing that is equal to a predetermined and fixed amplitude required by subsequent circuit blocks.
As shown, in a typical AGC loop, only one comparator is used to gather the amplitude information. However, using one comparator does not provide high resolution since only information for determining whether the output amplitude is above or below a desired signal level is provided by a single comparator. This does not supply information about how far away the output amplitude is from the desired signal level. Furthermore, since this typical scheme is coarse and nonlinear, it does not exhibit constant bandwidth tracking and step response.
To improve the resolution, a high precision analog circuitry, such as log domain amplifier is sometimes utilized to reach the goal of constant bandwidth step response for AGC circuit. See, for example, J. M. Khoury, “On the design of constant settling time AGC circuits”, Circuits and Systems II: Analog and Digital Signal Processing, IEEE Transactions, Volume 45, Issue 3, March 1998, p 283-294. However, since such a solution is analog, it has a high complexity and high power consumption.
Therefore, there is a need for an AGC loop which provides a balance between simplicity and accuracy compared to existing schemes.